In a liquid crystal display device, a classification based on an operating mode for liquid crystals includes a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, a fringe field switching (FFS) mode and a polymer sustained alignment (PSA) mode. A classification based on a driving mode in a device includes a passive matrix (PM) and an active matrix (AM). The PM is further classified into static, multiplex and so forth, and the AM is classified into a thin film transistor (TFT), a metal insulator metal (MIM) and so forth. The TFT is further classified into amorphous silicon TFT and polycrystal silicon TFT. The latter is classified into a high temperature type and a low temperature type according to a production process. A classification based on a light source includes a reflective type utilizing natural light, a transmissive type utilizing backlight and a transflective type utilizing both the natural light and the backlight.
The devices contain a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. General characteristics of the composition should be improved to obtain an AM device having good general characteristics. Table 1 below summarizes a relationship of the general characteristics between two aspects. The general characteristics of the composition will be explained further based on a commercially available AM device. A temperature range of the nematic phase relates to a temperature range in which the device can be used. A preferred maximum temperature of the nematic phase is about 70° C. or higher and a preferred minimum temperature of the nematic phase is about −10° C. or lower. Viscosity of the composition relates to response time in the device. A short response time is preferred for displaying moving images on the device. Accordingly, a small viscosity in the composition is preferred. A small viscosity at a low temperature is further preferred. An elastic constant of the composition relates to contrast in the device. In order to increase the contrast in the device, a larger elastic constant in the composition is further preferred.
TABLE 1General Characteristics of Composition and AM DeviceGeneral CharacteristicsGeneralNo.of CompositionCharacteristics of AM Device1wide temperaturewide usable temperature rangerange of a nematic phase2small viscosity1)short response time3suitable optical anisotropylarge contrast ratio4large positive or negativelow threshold voltage anddielectric anisotropysmall electric power consumptionlarge contrast ratio5large specific resistancelarge voltage holding ratio andlarge contrast ratio6high stability to ultravioletlong service lifelight and heat7large elastic constantlarge contrast ratio and short responsetime1)A liquid crystal composition can be injected into a liquid crystal cell in a shorter period of time.
An optical anisotropy of the composition relates to a contrast ratio in the device. A product (Δn×d) of the optical anisotropy (Δn) of the composition and a cell gap (d) in the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on a type of the operating mode. The suitable value is about 0.45 micrometer in a device having the mode such as TN. In the above case, a composition having a large optical anisotropy is preferred for a device having a small cell gap. A large dielectric anisotropy in the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio in the device. Accordingly, the large dielectric anisotropy is preferred. A large specific resistance in the composition contributes to a large voltage holding ratio and a large contrast ratio in the device. Accordingly, a composition having a large specific resistance at room temperature and also at a temperature close to a maximum temperature of the nematic phase in an initial stage is preferred. A composition having a large specific resistance at room temperature and also at a temperature close to the maximum temperature of the nematic phase after the device has been used for a long period of time is preferred. Stability of the composition to ultraviolet light and heat relates to a service life of the liquid crystal display device. In the case where the stability thereto is high, the device has a long service life. Such characteristics are preferred for an AM device used in a liquid crystal projector, a liquid crystal television and so forth. A large elastic constant in the composition contributes to a large contrast ratio and a short response time in the device. Accordingly, the large elastic constant is preferred.
A composition having a positive dielectric anisotropy is used for an AM device having the TN mode. On the other hand, a composition having a negative dielectric anisotropy is used for an AM device having the VA mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having the IPS mode or the FFS mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having the PSA mode. Examples of the liquid crystal compositions having the positive dielectric anisotropy are disclosed in the following Patent literatures.